Stent having flexibly connected adjacent stent elements

ABSTRACT

An open stent (a stent having open space through its thickness at locations between the ends of the stent), incorporating flexible, preferably polymeric, connecting elements into the stent wherein these elements connect adjacent, spaced-apart stent elements. Preferably the spaced-apart adjacent stent elements are the result of forming the stent from a helically wound serpentine wire having space provided between adjacent windings. Other stent forms such as multiple, individual spaced-apart ring-shaped or interconnected stent elements may also be used. The connecting elements are preferably longitudinally oriented.

FIELD OF THE INVENTION

The present invention relates to the field of implantable stents havingflexibly connected adjacent stent elements.

BACKGROUND OF THE INVENTION

The use of implantable stents in the vasculature and other body conduitshas become commonplace since first proposed by Dotter in the 1960's.These devices were required to have a small, compacted diameter forinsertion into the intended body conduit and transport, typically via acatheter, to a desired site for deployment, at which site they wereexpanded to a larger diameter as necessary to fit interferably with theluminal surface of the body conduit. They developed into balloonexpandable stents that were expanded by plastically deforming the devicewith an inflatable balloon on which the expandable stent was mounted inthe compacted state, the balloon being attached to the distal end of thecatheter and inflated via the catheter. Self-expanding stentssubsequently evolved, these devices being forcibly compacted to a smalldiameter and restrained at that diameter by a sleeve or other means.Following delivery to a desired site for deployment, they are releasedfrom the restraint and spring open to meet the luminal surface of thebody conduit. These devices are typically made from nitinol metal alloysand typically rely on the superelastic and biocompatible character ofthis metal. Nitinol stents that rely on the shape memory attributes ofthat material are also known.

The evolution of implantable stents included the use of a tubularcovering fitted to the stent, either to the outer or the luminal surface(or both surfaces) of the stent. These covered stents have generallycome to be referred to as stent-grafts. The coverings were generally ofa polymeric biocompatible material such as polyethylene terephthalate(PET) or polytetrafluoroethylene (PTFE). See, for example, U.S. Pat. No.4,776,337 to Palmaz. This patent also describes that the covering may beoptionally provided with perforations if desired for particularapplications. Because of the open area provided by the perforations,such devices having perforated coverings may be considered to be a sortof hybrid stent and stent-graft, as are devices that include stent framehaving metallic stent elements and polymeric elements connecting,covering or other otherwise being attached to the stent elements. Thepresence of the polymeric elements reduces the otherwise open spacebetween the adjacent metallic stent elements, either very slightly orvery substantially depending on the intended application and mechanicaldesign.

Generally, a fully covered stent-graft can be considered to have asurface area (hereinafter A_(max)) equal to the circumference of theexpanded stent multiplied by the length of the stent. For aconventional, open frame stent (as opposed to a stent-graft), thesurface area represented by all of the stent elements is only a smallportion of the maximum surface area A_(max). The actual surface areacovered by the stent, meaning the area covered by all components of thestent (including connecting elements) in their deployed state, isA_(stent). The porosity index, or P.I., describes the open area (theportion of the maximum surface area not covered by all components of thestent assembly) as a percentage of maximum surface area, wherein:

P.I.=(1−(A _(stent) /A _(max)))×100%.

The open area may be a continuous single space, such as the spacebetween windings of a single helically wound stent element. Likewise theopen area may be represented by the space between multiple individualannular or ring-shaped stent elements. The open area may also berepresented by the total area of multiple apertures provided by either asingle stent element (e.g., as shown by FIGS. 1B and 2B of U.S. Pat. No.4,776,337) or by multiple stent elements providing multiple apertures.If multiple apertures are provided they may be of equal or unequalsizes. The use of a perforated graft covering or of polymeric elementsin addition to metallic stent elements may also reduce the open area.

Stents having a porosity index of greater than 50% are considered to besubstantially open stents.

In addition to the porosity index, the size of any aperture providingthe open area must be considered if it is intended to cover only aportion of a stent area for a specific stent application. For multipleapertures, often the consideration must be for the largest size of anyindividual aperture, particularly if the apertures are to provide for a“filtering” effect whereby they control or limit the passage of biologicmaterials from the luminal wall into the flow space of the body conduit.

Various stent devices combining metallic stent elements with polymericconnecting elements are known; see, for example U.S. Pat. No. 5,507,767to Maeda et al. Another is a stent provided with a flexible knittedsleeve having small open apertures in the fashion of chain link fencing,from InspireMD Ltd. (4 Derech Hashalom St., Tel Aviv 67892 Israel).Perforated stent-grafts are also known; see, for example WO00/42949.

SUMMARY OF THE INVENTION

The present invention relates to several approaches to creating an openstent, that is, a stent having open space through its thickness atlocations between the ends of the stent, by incorporating flexible,preferably polymeric connecting elements into the stent wherein theseelements connect adjacent, spaced-apart stent elements. Preferably thespaced-apart adjacent stent elements are the result of forming the stentfrom a helically wound serpentine wire having space provided betweenadjacent windings. Other stent forms such as multiple, individualspaced-apart ring-shaped stent elements may also be used as will bedescribed, but embodiments presented that utilize the helically woundserpentine forms are preferred for many applications.

The adjacent, spaced-apart stent elements are substantiallycircumferentially oriented, meaning that they have a general directionof orientation perpendicular to the longitudinal axis of the stent, whenthe stent is in a straight (unbent state) form, plus or minus 45°.

The flexible, preferably polymeric connecting elements provide a meansfor keeping the stent elements equally spaced and allow the constructionof a stent having good flexibility. These flexible connecting elementsare preferably substantially oriented in a longitudinal direction withrespect to the stent, meaning that they are more longitudinally orientedthan circumferentially oriented. They may range in orientation frombeing perfectly parallel to the longitudinal axis of the stent (when thestent is in a straight, unbent form) up to an angle of 45° from thelongitudinal axis. More particularly, they may be oriented at angles ofless than or equal to about 45°, 40°, 35°, 30°, 25°, 20°, 15°, 10°, 5°,4°, 3°, 2° or 1° from the longitudinal axis, or may be virtuallyparallel to the longitudinal axis of the stent. Being parallel to thelongitudinal axis or close to parallel (e.g., +/−50) is preferred.

The described stents have very good porosity index values, typically atleast 50% and may be made to be at least 55%, 60%, 65%, 70%, 75%, 80%,85%, 90%, 95% or 97%. These stents can be considered to be substantiallyopen.

The flexible connecting elements may optionally provide a substrate forthe delivery of therapeutic agents such as drugs, which may be intendedfor any of a variety of medical purposes. Coatings for other purposes(e.g., to render the surface hydrophilic) may also be applied.

The stent may also be designed to allow it to be removed after itsprimary therapeutic effect has occurred. The stent may be removed byincluding elements that may be snared with an intravascular snare.

While polymeric materials are preferred as the connecting elements,other non-polymeric materials such as nitinol wire offer goodflexibility and may also be used.

A helically wound, undulating or serpentine metal wire stent structureprovides good flexibility and is preferred for providing the necessarystent structure having spaced apart stent elements. These structures andother stent structures may be used for self-expanding or balloonexpandable stents made respectively from (for example) nitinol orstainless steel. While metal wire is preferred for structures of thistype, they may also be made from metal tubing by various known machiningmethods.

While metal materials are preferred, the stent elements may optionallybe made of various polymeric materials that offer suitable physical andmechanical properties and may offer other unique properties desired forspecific applications (such as, for example, bioabsorbable polymers).

The first fundamental embodiment is a stent structure wherein at leastone substantially longitudinally oriented flexible, preferably polymericstrip is used to maintain spacing of adjacent stent elements, such asthe adjacent windings of a helical stent form. More than one such stripmay be used, with multiple strips (two or more) preferably being spacedapart equal circumferential distances around the circumference of thestent. While the orientation of the strip or strips is primarilylongitudinal, it may also have a helical component such that it is notparallel to the longitudinal axis of the stent when the stent is in astraight or ‘unbent’ state. The strip is substantially longitudinallyoriented if it is parallel to the longitudinal axis of the stent when itis in the straight or unbent state, plus or minus 45°. The use of thesubstantially longitudinally oriented strip results in a flexible stentwith good bending properties. The strip limits elongation of the stent,particularly during expansion of the stent from a compacted diameter toa fully deployed diameter. It also limits foreshortening of the stent,also particularly during expansion.

A second fundamental embodiment, which may also be based on thehelically wound serpentine wire form, uses a flexible, preferablypolymeric filament laced along the length of the stent to locate theadjacent stent elements (e.g., adjacent helical windings) with respectto each other. This filament is preferably laced so that it includes atransversely oriented loop around one half of a full sinusoid of theserpentine wire, with these loops created around sequential sinusoidsalong the length of the stent that are preferably axially aligned. Oneor more of these filament lacings may be provided along the length ofthe stent. When more than one lacing filament is used, they arepreferably spaced apart in equal circumferential amounts (e.g., if threelacing filaments are used along the length of the stent, they arepreferably spaced 120° apart).

A third fundamental embodiment includes a stent provided with aperforated covering of a flexible, preferably polymeric graft material,in the form of a sheet of material rolled and preferably seamed to forma tube, or alternatively as a seamless integral tube, that is providedwith a multiplicity of perforations or apertures. The covering materialis thus integral or monolithic as opposed to being created from separatefilaments, threads or other assembled components with multiple crossoverpoints that add to thickness and may be vulnerable to breakage andconsequent unraveling of the covering. The perforated covers describedare used to connect adjacent spaced apart stent elements and may also beused with a variety of stent forms including separate rings, helicallywound wires, machined metal tubes, etc. The perforated covering is thin,strong and flexible and, in combination with the stent elements, resultsin a flexible, thin and strong stent. Perforation sizes may be asdesired, with relatively small perforations on the order of 0.10 mm(minimum aperture size) being preferred for carotid applications whereit is desirable to minimize risk of introduction of emboli into abloodstream. Larger apertures (for example 1, 2 or even 5 mm) may bepreferred for larger diameter vessels (e.g., the thoracic arch) where itmay be desired to stent the arch (for example, in the treatment of adissection and/or containment of emboli) while allowing blood flow toside branches through the apertures of the graft material. Theseembodiments preferably have high porosity indices and preferablyincorporate hexagonal apertures, although other shapes are possible.

It is also possible to combine the different stent element connectingtechniques described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a perspective view of a preferred stent for use with thepresent invention.

FIG. 1A shows a plan view of details of serpentine wire forms describedby FIG. 1.

FIG. 2 describes a perspective view of an open-frame stent whereinspaced-apart, adjacent stent elements are interconnected by a flexiblestrip of implantable material.

FIGS. 2A-2D are transverse cross sectional views that show variousrelationships between connecting strips and stent elements.

FIG. 2E is a perspective view showing that a connecting strip may have aslight helical orientation while still being substantiallylongitudinally oriented.

FIG. 2F shows a longitudinal cross section of a stent provided with twolongitudinally oriented strips placed 180° apart.

FIG. 3A is a schematic view plan view of serpentine wire segments fromportions of four adjacent helically wound stent elements, showing asubstantially longitudinally oriented filament interconnecting adjacentstent elements.

FIGS. 3B and 3C are detail views of a portion of FIG. 3A showingparticulars of the filament relationship with the serpentine wire stentelements.

FIG. 3D shows a schematic plan view of an alternative serpentine wireform for a stent wherein the serpentine wire may be provided with acircumferential rather than helical orientation, and with stent elementsinterconnected by a substantially longitudinally oriented filament.

FIG. 3E is a detail view of a portion of FIG. 3D.

FIG. 3F shows the same stent form as FIG. 3D with an alternativearrangement of the interconnecting filament.

FIG. 3G shows an alternative stent form to that of FIG. 3F with the samefilament arrangement.

FIG. 3H shows an alternative arrangement of the substantiallylongitudinally oriented interconnecting filament.

FIG. 3I is a detail view of a portion of FIG. 3H.

FIG. 3J shows an alternative arrangement of the substantiallylongitudinally oriented interconnecting filament.

FIG. 3K is a detail view of a portion of FIG. 3J.

FIGS. 3L-3N show examples of filament arrangements that may be used withopposing apices.

FIG. 4A is a plan view of a helically wound serpentine wire stentprovided with a hexagonally perforated graft covering, as it wouldappear if the tubular form of the stent was cut along its length andlaid out flat.

FIG. 4B is a plan view of a series of individual serpentine stent rings,arranged with the direction of the apices opposing each other ratherthan aligned in the same direction as shown in FIG. 4A, provided with ahexagonally perforated cover.

FIG. 4C is a transverse cross section of a stent 10 provided with theperforated covering 40.

FIGS. 4D-4G describe plan views of alternative perforated coveringshaving alternatively shaped perforations.

FIG. 4H is a perspective view of a stent provided with a perforatedcover shown as it would appear deployed in vivo, located in a vessel ata site where the stent crosses a side vessel and with a balloonintroduced through a cover perforation and extending into the sidevessel.

FIGS. 4I and 4J show stents provided with perforated coverings placedover thoracic aneurisms.

FIG. 4K shows a stent provided with a perforated cover used to repair adissection in a blood vessel.

FIG. 4L shows a stent provided with a perforated cover used to holdplaque against a vessel wall while still allowing flow through anadjacent side branch vessel.

FIG. 4M shows a stent provided with a perforated cover used to holdplaque or emboli contained against the wall of a small blood vessel.

FIG. 4N shows a stent provided with a perforated cover located distallyto a stent-graft located to repair an aneurysm wherein the perforatedcover stent aids in anchoring the stent-graft near a branch vessel whileallowing flow through the branch vessel.

FIG. 4P is a schematic side view of a stent tensile test configuration.

DETAILED DESCRIPTION OF THE DRAWINGS

Several open frame stent designs are presented, wherein adjacent stentelements are connected by flexible connecting elements. These flexibleconnecting elements are typically polymeric and may take various formssuch as strips, filaments or perforated sheets. Typical stent forms arehelically wound metallic wire (e.g., nitinol or stainless steel) ormultiple ring-shaped metallic stent elements. The helically wound wireis preferably serpentine wire as will be further described. Theserpentine wire may also be oriented circumferentially in the fashion ofindividual rings or may alternatively be a single continuous wirearranged as will be described. In addition to wire forms, various otherstent forms, typically metallic but not limited to metallic, may also becreated that lend themselves to the present invention. Among these aremachined tubular forms wherein individual ring-shapes, or alternativelyperforated tubes or other forms (e.g., helical) that extend continuouslybetween opposing ends of the stent.

FIG. 1 shows a perspective view of a stent 10 that is preferred for usewith the present invention. The stent 10 shown comprises a helicalwinding of a length of serpentine wire 18. Sequential windings of thehelical wound serpentine wire 18 result in spaced-apart adjacent stentelements 12. The ends 17 of wire 18 may be secured by any suitablemethod (e.g., welding) to the adjacent helical winding. For clarity,stent 10 is shown with a mandrel 16 extending through and beyond bothends of the stent lumen, making the side closest to the viewer visuallyapparent while blocking the view of the side of stent 10 furthest fromthe viewer. Mandrel 16 is present only for clarity of visualization andis not a part of stent 10.

The helically wound serpentine wire 18 extends continuously betweenopposing ends of stent 10, wherein opposing apices 22 a and 22 b formedof wire bends of relatively small radii are interconnected by straightor relatively straight wire segments 24. The apices typically “point” indirections that are substantially parallel to the longitudinal axis 19of the tubular form of the stent 10, with alternating apices 22 a and 22b pointing in opposite directions, that is, pointing to opposite ends ofthe stent. As shown by FIG. 1, it is preferred that apices pointing inone direction (e.g., apices 22 a) are aligned along a first common linewhile the apices pointing in the opposite direction (e.g., apices 22 b)are aligned along a second common line.

FIG. 1A shows a plan view of details of serpentine wire forms describedby FIG. 1; dimensions relate to Example 1 described below. Dimension 27is considered as the height of adjacent opposing apices while dimension28 is the width of adjacent opposing apices. Dimension 29 describes onefull period of the serpentine form.

FIG. 2 describes a perspective view of an open-frame stent 10 whereinspaced-apart, adjacent stent elements 12 are interconnected by aflexible strip 14 of implantable material. Adjacent stent elements 12may be individual ring-shaped elements or alternatively adjacentwindings of a continuous helically wound wire. The use of connectingstrips 14 provides the resulting stent with good flexibility and goodaxial strength while maintaining the spacing of the adjacent stentelements 12.

Connecting strips 14 are preferably polymeric, and may be made from avariety of implantable polymeric materials including bioabsorbablepolymers. Bioabsorbable polymers particularly lend themselves to theconstruction of stents having flexibly connected stent elements whereinthe stent may be removed (particularly a helically wound stent) afterdegradation of the bioabsorbable connecting elements. ePTFE strips(porous expanded PTFE) made from ePTFE films are particularly preferredfor their strength, flexibility and biocompatibility.

Strips may extend for less than the entire length of the stent ifappropriate. If more than one strip is used, the individual strips mayhave different lengths or the same length. It is generally preferred,however, that all strips extend for the full length of the stent.

Strips 14 are joined to stent 10 at points where they are in mutualcontact. It is preferred that all such contact points are attached.Preferably they are attached by the use of suitable adhesives or bymelt-bonding of the strip polymer. Various biocompatible adhesives maybe used, including melt-bondable thermoplastics.

For ePTFE strips made from ePTFE films, a preferred adhesive is acontinuous coating of a thermoplastic fluoropolymer, particularlyfluorinated ethylene propylene (FEP). The FEP coating may be applied tothe ePTFE film by a process which comprises the steps of:

-   a) contacting one side of the ePTFE film with a layer of FEP film    (or another alternative thermoplastic polymer if so desired);-   b) heating the composition obtained in step a) to a temperature    above the melting point of the thermoplastic polymer;-   c) stretching the heated composition of step b) while maintaining    the temperature above the melting point of the thermoplastic    polymer; and-   d) cooling the product of step c).

The thermoplastic film coating applied to the ePTFE film by this methodmay be either continuous (non-porous) or discontinuous (porous). Ifdiscontinuous, the process may be adjusted to achieve the desired degreeof porosity to include a coated film that is as porous as the precursorePTFE film. The coated film used for the present invention is mostpreferably a continuously (non-porous or substantially non-porous)coated film.

While a single strip 14 may be used along only one side of the stent 10,oriented in a direction substantially parallel to the longitudinal axisof the stent, multiple (at least two) such strips are preferred, withthe strips preferably being equally spaced radially about the stent.Three connecting strips 14 are preferred to achieve uniform bendingproperties, as shown by the transverse cross section of FIG. 2A. Ideallythese are spaced equidistant radially (120° apart) around thecircumference of the stent, although some variability of this spacinghas not been seen to be particularly detrimental to bending properties.Strips 14 are shown attached to the outer surface of stent 10.Alternatively, strips 14 may be attached to the inner surface of stent10 (FIG. 2B) or to both the inner and outer stent surfaces (FIGS. 2C and2D).

As shown by FIG. 2E, strip 14 may have a slight helical orientationwhile still being substantially longitudinally oriented, althoughorientations parallel to the longitudinal axis are preferred.

FIG. 2F shows a longitudinal cross section of a stent 10 provided withtwo strips 14 placed 180° apart, with the section being taken througheach of the two strips 14. This figure shows how each strip 14 is placedon the inner surface of stent 10, with ends of the strip 14 extendingbeyond the ends of the stent. The strip 14 in this embodiment is inexcess of twice the length of the finished stent 10, allowing the endsof strip 14 to be wrapped back over and attached to the outer surfacesof the stent elements 12, with the extreme ends of strip 14 overlappingand attached to each other (reference 25). As well as strips 14 beingattached (bonded) to their points of contact with stent elements 12,they are attached to each other between adjacent stent elements as shown(reference no. 26). The ends of the stent 10 may optionally be providedwith a circumferentially applied tape covering 23 that aids in securingthe extreme wire ends of the wire stent.

A self-expanding stent was made using nitinol wire of round crosssection and of 0.15 mm diameter. The wire was wound into the helicallywound serpentine form (shown in FIG. 1) on a manufacturing mandrelhaving a series of pins protruding from its external surface at specificpoints. The resulting serpentine form had apices of about 0.3 mm radius(measured from the center point of the bend to the surface of the wireclosest to the center point of the bend). The height of adjacentopposing apices was about 1.6 mm (dimension 27 in FIG. 1A) while thewidth of adjacent opposing apices was about 2.3 mm. The resulting stentincorporated eight full periods of the serpentine pattern for each fullrevolution of the serpentine wire about the stent circumference. Thestent was of about 6 mm inside diameter, having been manufactured to itsfully deployed diameter at which the connecting strips were to beattached. The completed stent may subsequently be compacted to a smallerdiameter for insertion into a vasculature by various known meansincluding iris type compaction devices or by pulling the stent throughfunnel devices.

Following forming of the nitinol wire into the helically wound stent,the finished wire form, while still on the manufacturing mandrel, wasthen placed into a convection oven set at 450° C. for 9 minutes for heattreating, and removed from the oven and allowed to cool to ambienttemperature. The stent was then removed from the mandrel.

Connecting strips for this 5 cm long stent were 1.0 mm wide, 11 cm longstrips of ePTFE film of about 0.012 mm thickness subsequently providedwith a continuous (substantially non-porous) FEP coating. The ePTFE usedto make these strips was of about 0.5 g/cc density and of about 50micron average fibril length. The coated strip was of about 0.05 mmthickness. Break strength for these strips was 0.5 kg or greater.

Average fibril length of the ePTFE film was estimated from scanningelectron photomicrographs of the surface of the film. Film thicknessmeasurements are preferably made (including for the determination ofbulk volumes for density values) by placing a sample between the pads ofa Mitutoyo model no. 804-10 snap gauge having a part no. 7300 frame andgently easing the pads into contact with the opposing surfaces of thesample under the full force of the spring-driven snap gauge pads.

Three strips were used to create the stent, spaced apart radially. Eachof the three strips was aligned longitudinally (parallel to thelongitudinal axis of the stent) with and covering a row of wire apicespointing at the same end of the stent. Arrangement of the strips wassuch that between two strips was one row of uncovered apices pointing inthe same direction as the rows covered by the two strips. The other twospaces between the connecting strips each had two rows of uncoveredapices pointing in the same direction as the covered apices.

The stent having substantially longitudinally oriented connecting stripswas manufactured using the helically wound wire stent and FEP coatedePTFE connecting strips described above. The manufacturing processinvolved fitting a sacrificial 5 mm inside diameter, longitudinallyextruded and expanded ePTFE tube onto a porous metal 5 mm diameterstainless steel mandrel. Two circumferential wraps of 6 mm wide FEPcoated ePTFE film were applied over the surface of the sacrificial tubewith the FEP facing away from the sacrificial tube. These two wraps werelocated about 4.4 cm apart. Three of the 11 cm long FEP coated stripswere laid lengthwise along the surface of the sacrificial ePTFE tubewith their FEP coated surface facing away from the sacrificial tube.These strips were spaced approximately 120° apart circumferentially. Theends of the strips were temporarily secured to the mandrel with ePTFEtape. The helically wound 5 cm long serpentine wire stent was thenfitted over the assembly with the stent ends centered on eachcircumferential wrap of 6 mm wide ePTFE film. Care was taken to see thatthe adjacent helical stent windings were spaced apart equal distances.After removing the temporary ePTFE tape, the radial position of each ofthe three ePTFE connecting strips was then adjusted with respect to thestent so that each strip was located along a row of stent apices pointedin the same direction for all three strips.

For each strip, both ends were laid back over the outer surface of thestent in line with the portion of the strip beneath the stent, withabout 0.5 cm of the very ends of both strips overlapping as shown byreference no. 25 in FIG. 2F. The strips were secured in this positionwith a temporary wrapping of 0.012 mm thick polyimide film. Each stentend was then given a circumferential wrapping with a narrow gold markerband (e.g., 0.0625 mm by 0.025 mm) and an outer circumferential wrappingof another layer of 6 mm wide FEP coated ePTFE film, this time with theFEP facing inward. The entire assembly was wrapped in polyimide film andplaced into a convection oven set at 320° C. for 10 minutes with avacuum applied to the porous metal mandrel. After removal from the ovenand being allowed to cool to about ambient temperature, the polyimidefilm was removed. Any of the circumferentially wrapped ePTFE filmcovering the stent ends that protruded beyond the ends of the wire stentwas trimmed off with a scalpel.

The three ePTFE connecting strips were well adhered to the stent and tothemselves between the stent elements (reference no. 26, FIG. 2F). Theresulting stent had a porosity index of about 82%. It demonstrated goodflexibility and kink resistance in bending. The stent was able to bebent to an inside bend radius of 3 mm without kinking.

The same stent made with a single connecting strip of the samedimensions would have a porosity index of 93% A second fundamentalembodiment uses a flexible, preferably polymeric filament laced alongthe length of the stent to locate the adjacent stent elements (e.g.,adjacent helical windings) with respect to each other. Stent elementsinclude serpentine wire or machined elements having repeating sinusoidswith apices connecting relatively straight segments. The stent elementsmay be individual rings or may be adjacent windings of a helically woundserpentine wire, or may be any other stent form having spaced apartelements that lend themselves to being connected by a filament.Alternate apices of the serpentine form point in opposite directions,i.e., toward opposite ends of the stent. The filament may be of avariety of polymers including PET, polyurethane, PTFE, etc. Porousexpanded PTFE fibers are preferred because of their strength,flexibility and biocompatibility. The filaments may optionally beprovided with coatings of adhesives (including heat-activated adhesives)to allow bonding to stent elements. The filaments may also be abioabsorbable polymer such as PGA, PLA, PGA/PLA, PGA/TMC, etc. The useof a bioabsorbable filament is possible in that, once deployed at adesired site in a body conduit, the interference of the deployed stentelements with the luminal surface of the body conduit will hold thestent in place with the elements properly spaced apart for most implantapplications.

The filament is substantially longitudinally oriented (along the lengthof the stent) as it connects adjacent, spaced-apart stent elements,meaning that the predominant orientation of the filament, represented bya straight line (neglecting the curvature of the exterior surface of thestent, i.e., as if the stent form were considered in a flattened, planview) laid over the filament between the ends of the stent, issubstantially longitudinally oriented. This line is consideredsubstantially longitudinally oriented if it is parallel to thelongitudinal axis of the stent, plus or minus 45°. Most preferredorientations are parallel to the longitudinal axis, or very close toparallel (plus or minus 5°). As shown by the schematic plan view of FIG.3A, the filament is preferably laced so that it includes a transverselyoriented loop around one half of a full period of the serpentine wire,with these loops created around sequential periods of the serpentineform that lie along the length of the stent in axial alignment. One ormore of these filament lacings may be provided along the length of thestent. When more than one lacing filament is used, they are spaced apartin equal radial amounts (e.g., if three lacing filaments are used alongthe length of the stent, they are spaced 120° apart). The use of threefilaments offers a particularly good combination of stent flexibilityand strength.

FIG. 3A is a schematic view plan view of serpentine wire segments fromportions of four adjacent helically wound stent elements 12incorporating opposing apices 22 a and 22 b connected by straightsegments 24. These spaced-apart stent elements 12 are interconnected byfilament 30 extending between both ends (not shown) of stent 10.Filament 30 is laced through stent elements in the fashion shown so thatit includes longitudinal segments 30 l and transverse loop segments 30t. This form of lacing provides very good stent flexibility and axialstrength while allowing a degree of length variability in that theapplication of tension to the length of the stent results in a slightamount of length extension. The pattern of the lacing shown is bestfollowed by examination of the close-up details of FIGS. 3B and 3C, witharrows 30 a, 30 b and 30 c showing the lacing accomplished in thatrespective sequence.

FIG. 3D shows a schematic plan view of an alternative to the helicallywound wire stent of (for example) FIGS. 1 and 3A. The sinusoids shown inFIG. 3D include opposing apices 22 a and 22 b connected by straightsegments 24 in the fashion shown previously for the helically woundconstructions. The stent portion shown in FIG. 3D differs in that, onceper revolution, straight segment 24 is replaced by a longer straightsegment 32 that connects one winding (or stent element) 12 to theadjacent winding (or stent element) 12. This use of the longer straightsegments 32 allows the individual windings 12 to be circumferentiallyoriented (perpendicular to longitudinal axis 19) rather than helicallyoriented wherein the windings have a pitch that is less thanperpendicular to the longitudinal axis 19. This stent arrangement may beused with connecting strips 14 in a fashion similar to that shown inFIG. 2, and for other embodiments as will be subsequently described.FIG. 3D and the detail of FIG. 3E show the transverse segment of thelacing filament 30 t to extend around the adjacent air of longerstraight segments 32 as well as one adjacent straight segment 24 ofconventional length. Any other filaments extending between ends of thestent (e.g., two other filaments if a total of three are used) are lacedin the fashion shown by FIG. 3A-3C.

Alternatively, as shown by FIG. 3F, the transverse loop 30 t may befitted only around one longer straight segment 32 and the adjacentstraight segment 24 of conventional length, thereby passing between theadjacent long straight segments 32.

FIG. 3G shows a similar stent structure to that of Figure F, differingprimarily in that the long straight segments 32 are offset by one periodof the serpentine form per each circumferential revolution. The filamentlacing pattern is the same.

An additional advantage of the lacing patterns shown in FIGS. 3A-3G isthat manufacture of these patterns may be easily automated.

Other lacing techniques are also possible, such as those shown by FIG.3H and associated detail FIG. 3I, and FIG. 3J and associated detail FIG.3K. FIGS. 3L-3N show examples of how the lacing techniques may beapplied to stents having opposing apices. FIG. 3N shows an embodimentwherein two or more filaments are twisted together, capturing the stentelement as the twisting proceeds.

It is apparent that a variety of substantially longitudinally orientedfilament lacing methods may be used with various stent forms havingspaced-apart stent elements, each method having advantages. For example,substantially longitudinally oriented patterns that also incorporatetransverse filament lacing aspects (for example, of the type describedby FIG. 3A) can be used to affect torsional properties of the stent.

As noted above, the chosen filaments may be provided with adhesives ifdesired for attachment of contact points of the filaments to the stentelements. While this may inhibit stent flexibility, it may be desiredfor other reasons such as precisely limiting the length of an expandedstent. ePTFE filaments may be provided with melt-bondable coatings ofpolymers having lower melt temperatures than PTFE; fluorinated ethylenepropylene (FEP) is an example. Such a coating may be applied by variousmethods including extrusion over the filament, powder coating of thefilament with powdered FEP that is subsequently melted to flow over thefilament surface, or running the filament through a bath of molten FEPoptionally followed by pulling the filament through a die to achieveuniformity of the coating. Alternatively, the stent may be provided witha coating of adhesive such as by powder coating with FEP. ePTFEfilaments may be made by rolling ePTFE films (see, for example, U.S.Pat. No. 5,288,552 to Hollenbaugh et al.). The films used to createfilaments may be FEP coated films of the type described previously.

An example of a 6 mm expanded diameter nitinol wire stent havingadjacent elements connected by filaments was created using the same typeof helically wound serpentine wire stent as created for Example 1. Thefilament chosen was an ePTFE filament provided with a coating of FEPmelt-bondable adhesive. The filament had a diameter of 0.1 mm, a tensilestrength of 620 grams and a weight per unit length of 0.018 grams/meter.Eight filaments were laced into the stent, connecting the adjacent stentelements as shown by FIGS. 3A-3C, with one filament laced into each rowof apices that pointed at one end of the stent. The resulting stentexhibited excellent flexibility, being able to be bent to an inside bendradius of almost zero without kinking. Porosity index for this stent was97%.

A third fundamental embodiment is a stent provided with a perforatedcovering of a thin, strong and flexible, preferably polymeric graftmaterial, in the form of a sheet of material rolled and preferablyseamed to form a tube, or alternatively as a seamless integral tube,that is provided with a multiplicity of perforations or apertures. Thegraft material is thus integral or monolithic as opposed to beingcreated from separate filaments or threads with multiple crossoverpoints that add to thickness and may be vulnerable to breakage andconsequent unraveling of the covering. Various implantable polymericmaterials may be used for the perforated cover including PTFE, PET,polyurethane, silicone, fluoroelastomers and bioabsorbable polymers.

FIG. 4A is a plan view of a helically wound serpentine wire stent 10provided with a perforated graft covering 40, as it would appear if thetubular form of the stent was cut along its length (parallel to thelongitudinal axis) and laid out flat. The stent is the same type asshown in FIG. 1. The hexagonally perforated covering 40 is a preferredcovering offering good flexibility and strength when made as describedbelow.

FIG. 4B is a similar plan view of a series of individual serpentinestent rings 12 wherein two rings per row are connected by stentelements, arranged with the direction of the apices opposing each otherrather than aligned in the same direction as shown in FIG. 4A; the stentof FIG. 4B uses the same hexagonally perforated graft covering 40 as thestent of FIG. 4A.

FIG. 4C is a transverse cross section of a stent 10 provided with theperforated covering 40. The stent elements 12 shown are of wire having arectangular cross section. Stent elements 12 are provided with a coatingof an adhesive 41 such as a melt-bondable FEP applied to the stentelements 12 by powder coating.

FIGS. 4D-4G describe plan views of alternative perforated coveringshaving different hole patterns. While hexagonal perforations arepreferred, the other perforation patterns such as rectangular, circular,triangular and square may be desirable for certain applications. It isalso apparent that, for perforation patterns that are not perfectlysymmetrical within the plane of the covering, that the covering sheetmay be oriented as desired with respect to the length of the stent.Perforations may be made to desired sizes, with aperture size beingdefined herein as the diameter of the largest inscribed circle that maybe fitted into the opening. It is further apparent that different sizeopenings may be provided within the same covering. The total openingarea and the amount of web material between openings may be selected toprovide the desired porosity index.

The rectangular patterns of FIG. 4D may also be made by using slitsthrough the material. Either the rectangular or slit pattern may beusefully oriented with the long dimensions of the slits or rectanglesparallel to the longitudinal axis of the stent. Openings of this typemay allow the covering to be attached to a stent in a partially or fullycompacted state, wherein, upon expansion of the stent, the slit orrectangle deforms into a larger generally hexagonally shaped aperture.

FIG. 4H is a perspective view of a stent 10 provided with a perforatedcover 40 shown as it would appear deployed in vivo, located in a vesselat a site where the stent 10 crosses a side vessel (not shown). Aguidewire 42 is introduced through a perforation aligned with theentrance to the side vessel, after which a balloon catheter isintroduced over the guidewire directing the balloon 44 through the sameperforation. The balloon 44 may subsequently be inflated to rupture theperforation and create an enlarged transmural opening through theperforated graft cover aligned with the entrance to the side vessel.

FIGS. 4I and 4J show stents 10 provided with perforated coverings 40implanted into blood vessels 41 over thoracic aneurisms 46. Theaneurisms 46 may be filled with a suitable gel or other biocompatiblematerial that is held in place by perforated cover 40 while blood flowis provided through the lumen of the stent 10. Blood flow is alsomaintained into side branch vessels 48 through the perforated cover 40.

FIG. 4K shows how a stent 10 provided with a perforated cover 40 may beused to repair a dissection 49 in a blood vessel 41.

FIG. 4L shows a stent 10 provided with a perforated cover 40 used tohold plaque 52 against a vessel wall while still allowing flow throughan adjacent side branch vessel.

FIG. 4M shows a stent 10 provided with a perforated cover 40 used tohold plaque or emboli 52 contained against the wall of a small bloodvessel 48.

FIG. 4N shows a stent 10 provided with a perforated cover 40 locateddistally to a stent-graft 54 located to repair an aneurysm wherein theperforated cover stent aids in anchoring the stent-graft near a branchvessel while allowing flow through the branch vessel. The aneurisms mayoptionally be filled with coils, a suitable gel or other biocompatiblematerial.

FIG. 4P is a schematic side view of a stent tensile test configuration.

Perforated covers were created by initially wrapping several layers ofan ePTFE film that includes a discontinuous (porous) layer of FEP. Filmsmade as taught by U.S. Pat. No. 5,476,589 to Bacino are suitable for FEPcoating and use in this application. The film used ranged from 2.5 to 5microns in thickness and had a density range of about 0.5 to 1.0 g/cc.The film was wrapped circumferentially, with the FEP side orientedoutwards, onto a glass mandrel approximately 1 mm diameter larger thanthe outside stent diameter. Other materials, including biocompatiblepolymers and metals could be used for the perforated cover structure,with process parameters adjusted accordingly. Twelve layers of the filmwere wrapped around the mandrel surface, with a range of 2 to 100 layersconsidered desirable. The wrapped mandrel was placed in a convectionoven set at 320° C. for 12 minutes, and then allowed to cool to aboutambient temperature.

While the perforations may be formed by various methods including theuse of, for example, mechanical punches, laser cutting is preferred forspeed and precision.

For cutting the perforations, the wrapped mandrel was set up on acomputer controlled laser cutting tool that utilizes a beam with awavelength of 10.6 μm (Keyence ML-G9310, Woodcliff Lake N.J.). Shorterwavelengths lasers have been tried (e.g., 157 nm wavelength laser) withthe cut quality being higher (straighter cuts with less thermallyeffected zone as evidenced by less material retraction when visuallyinspecting scanning electron microscope images). The laser wasprogrammed to cut hexagonal apertures with side length of 0.15 mm.Adjacent hexagons were offset in honeycomb fashion to minimize theamount of material between the resulting apertures and to providerelatively uniform web widths between adjacent apertures. Accounting forthe laser beam width of 50 microns, the side length of the resultinghexagonal aperture is about 0.2 mm. Depending on the intendedapplication of the stent, the length of the cut side of the hexagons mayrange from 0.025 to 5 mm, with 0.1 mm to 1 mm being preferable. Otheraperture shapes of widely ranging sizes may also be cut. Theperforations may be made to be of uniform shape, or not. It is alsoanticipated that the perforations may be cut after attaching the coverto the stent.

After cutting, the wrapped mandrel was heated in a convection oven setat 370° C. for 5 minutes. This post-cutting heating step has the benefitof both improving the cut quality (i.e., smoothing the edge) andminimizing the width of the membrane between the cut hexagons. Thisheating process encourages retraction of polymer thereby narrowing themembrane width, and may also result in a thickness increase. Forexample, the difference between pre-heated to post-heated web width hasbeen measured to change from approximately 0.20 mm to 0.075 mm.

After the heating step, the resulting perforated stent cover wasstripped from the glass mandrel and inverted so the FEP that was on theouter surface became the inner surface. Optionally, radiopacityenhancements could be added to the perforated cover such as attachinggold foil segments to the cover by, for example, the use of a suitableadhesive or by locally melting the FEP (for example, with a heatedsoldering iron).

Following manufacture of the perforated cover, a suitable stent isobtained. The stent is preferably made of nitinol, but can be fabricatedof a material such as stainless steel, cobalt chromium, or bioabsorbablematerials (e.g., polyglycolic acid, or other). The stent may optionallybe provided with radiopaque enhancements such as gold orplatinum/iridium markers crimped, embossed, or otherwise attached to thestent frame. Various stents forms were attached to perforated coversmade as described. 6 mm stents of the type used to make the stripconnected and the filament connected stents were used, differing only inbeing of 3 cm length. 37 mm helically wound wire stents were made witheight full periods of the serpentine wire forms per circumference(reference no. 29, FIG. 1A), and the width of adjacent opposing apices(reference no. 28, FIG. 1) being equal to about 6.7 mm. The height ofadjacent opposing apices (reference no. 27, FIG. 1A) being equal toabout 9.5 mm. 37 mm stents were heat treated on their manufacturingmandrels in a convection oven set at 470° C. for 20 minutes. After beingremoved form the oven and allowed to cool to ambient temperature, theresulting stent was removed from the mandrel.

The obtained stent was powder coated with a thin layer of FEP powder.This was done by using FEP powder (Dupont FEP Fluoropolymer Resin,Product Type 5101) in a table top blender within which the stent issuspended. Other melt processable polymers could be used, includedfluoroelastomers, drug eluting polymers, or other polymers. The stentwas placed within the blender with FEP powder and the blender activated.The powder dispersed into the volume of the blender chamber and powdercoated the stent. After approximately 3 seconds, the stent was removed,and next placed into a convection oven set at 320° C. for 5 minutes.After this time, the stent was removed and allowed to air cool.

The stent was then placed on a mandrel having an outer diameterapproximately equal to the inner diameter of the stent. The mandrel wascovered on its outer diameter with polyimide film. To temporarily fixthe stent to the mandrel, the stent was placed in a convection oven setat 320° C. for 4 minutes.

After removal from the oven and cooling of the stent and mandrelassembly, the perforated cover structure was coaxially positioned overthe stent. The perforated cover was axially tensioned over the stent,causing it to decrease in diameter and come in full contact with theouter diameter of the stent. The cover ends were temporarily fixed tolength on the mandrel by ePTFE tape. A temporary layer of ePTFE film wasthen tightly wrapped around the assembly. The perforated cover was thenplaced within a convection oven set at 320° C. oven for 12 minutes.After removal from the oven and being allowed to cool to ambienttemperature, the temporary film wrapping was removed and the stent andperforated cover assembly removed from the mandrel. The perforated coverwas then trimmed flush with the end of the stent.

The perforated cover structure may also be attached by mechanical meanssuch as fiber or discrete mechanical attachment points (e.g., clips,etc). The perforated cover may be on the outside of the stent elements,or it may be on the inside of the stent elements, or it may be on both.

The resulting assembly should then be inspected to ensure good adherenceof the stent to the perforated cover. This final assembly can then becooled below its martensitic temperature, crimped and loaded within acatheter delivery system for implantation into a body conduit followingsterilization.

Assembling a covered stent in the preceding manner has a number ofadvantages. First, the radial strength of the device can be optimizedindependent of device flexibility and perforation size. By attaching theperforated cover structure, the axial distance separation of stent rowson the outer radius of curvature is minimized and bending isaccomplished by reducing the space between the adjacent stent elementson the inner radius of curvature. This allows stents to be constructedthat give more uniform support to a curved vessel.

The above-described perforated cover structure minimizes the amount ofmaterial covering the vessel lumen. This minimal material is anticipatedto allow the tissue to heal faster around and over the stent. Theporosity index of the stent and perforated cover may be quite high,comparable to that of conventional stents alone. This can beaccomplished by minimizing the amount of material with the structuralportion of the stent. Material can be removed because the traditionalmetal portion of the stent does not need to be optimized for vesselscaffolding, bending uniformity, or other conventionally consideredattributes. The coverage of the vessel luminal surface (including theperforated cover and metal stent) is preferably less than 50%, with lessthan 40%, 30%, or even 20% being possible and usually preferred. Thesenumbers correspond to porosity indices of 50%, 60%, 70%, and 80%respectively.

For the stent provided with the perforated cover, the luminal areacovered by the combined stent and cover may be determined using a stentinspection system (e.g., Visicon Finescan, Napa Calif.). Using theinspection system, a 1.0 cm length of the cylindrical device is imagedas a 360 degree flat pattern. The contrast of the image should besufficient to allow full visualization of the perforated structure andstent. From the flat pattern image, the area of coverage of the stentand perforated cover can then be determined. The percentage of porosityis then determined using the porosity index equation presented above.

The perforated cover provides a favorable substrate to deliver drugs orother therapeutic agents to the vessel. Because the perforated cover hasuniformly sized perforations, the elution of the drug into the vesselcan be controlled more precisely and uniformly.

The perforated cover structure creates uniform support of the vessel.The opening size in traditional stents or stents covered with a wovenmaterial can vary depending on how the stent is bent, on the diameter itachieves within a stenotic vessel, or other factors.

The structure is also highly reliable. For stents constructed with awoven covering, a single broken fiber may unravel and protrude into thevessel lumen. With the perforated cover structures described herein, allweb locations are supported by multiple additional webs in closeproximity, minimizing the potential for a broken web to protrude intothe lumen of the vessel. In addition, the web is fully attached to thestent minimizing the risk of material protruding into the lumen.

Another desirable aspect of the perforated cover is it allows continuedaccess to side branch vessels. A 0.35 mm diameter guide wire can beeasily threaded through the wall of the stent with a perforated coverpattern with perforations on the order of about 0.5 mm smallesttransverse dimension. This perforation can then be dilated up using aninterventional dilator to 1 millimeter diameter. Exchanging a balloonfor the dilator, and positioning the balloon through the perforatedcover over the guide wire, the perforated cover can be dilated to thedesired size. This process opens a transmural hole that would allowpassage of other devices (e.g., balloons, stents, etc) into the sidebranch and minimizes the disruption of the blood flow to the side branchartery.

The stent device shall have sufficient strength to allow, afterdeploying a partial length of the stent device, the stent device to berepositioned without suffering damage. These forces shall also begreater than the minimum forces required to break the delivery catheter,which may be as defined in ISO 10555-1 as 3N, 5N, 10N, or 15N (dependingon the diameter of the catheter). Correspondingly, for the 5 holes percircumference samples with 5 intermediate webs between the holes (SampleB1 and B2 described below), the minimum web tensile strength would bethe ISO 10555 values divided by 5 webs, or 0.6N, 1N, 2N, or 3N.

Device tensile strengths were measured in a tensile testing machine. 6mm and 37 mm diameter stents made as described above were clamped insemi-circular jaw inserts, as shown in FIG. 4P. The jaw inserts weremade from stainless steel and had a thickness of approximately 3 mm. Thewidth of the semi-circular cut in the jaw inserts 56 was 5 to 15% largerthan half the stent device's circumference; for example, for the 6 mmdiameter stent, a 10 mm semi-circular cut jaw insert width was usedwhile for the 37 mm diameter stent, a 65 mm semi-circular jaw insertwidth was used. This sizing allowed the ends of the device to beflattened and easily fit within the semi-circular feature of the jawinsert 56. This type of jaw insert configuration was selected tomaximize the potential for the sample to break away from the clamped jawinsert 56. Data from breaks that occur immediately adjacent to a jawedge are to be discarded. Gauge length 59 was measured from the top ofthe semi-circular cut in the top jay insert to the bottom of thesemi-circular cut in the bottom jaw insert, as shown in FIG. 4P. Tominimize the risk of jaw slippage, the jaw inserts 56 were lined ontheir inward face with 400 grit sandpaper using double-faced transparenttape. The edge of the semi-circle was also provided with a 1.5 mmradius. These semi-circular jaw inserts 56 were placed within thetensile tester's standard serrated jaws 58, as shown in FIG. 4P. The airpressure to the jaws was set to 0.62 MPa. The tensile tester (Instronmodel number 5564, Instron Corp., Norwood Mass.) was programmed for arate of 100% gauge length per minute (rate consistent with ISO 7198requirements). Using this requirement, the 6 mm diameter stents weretested using a gauge length of 15 mm with a rate of 15 mm/min, while the37 mm diameter stents were tested using a 40 mm gauge length with a rateof 40 mm/min. All testing occurred at room temperature (24° C.). Devicetensile strength was determined at the peak force from initial breakage(away from the jaw inserts) of the perforated cover.

Tensile strength of the web between a pair of adjacent perforations wasmeasured using a fiber was threaded through each of the perforations andaround the intervening single web of the perforated cover, as follows.On the bottom jaw of the tensile tester, a rod smaller than the diameterof the stent (i.e., 4.6 mm diameter stainless steel rod) was clamped inthe tensile tester's bottom jaw in a horizontal orientation. The stentto be tested was slid onto the rod and against the tensile tester's jaw.A length of fishing line (of about 0.35 mm diameter) was threaded arounda single web of the perforated cover by passing the line through twoperforations adjacent to the web, in one direction for the firstperforation and the opposite direction for the second perforation. Bothends of the fishing line were clamped to the upper jaw of the tensiletester with the air pressure supplied to operate the jaws set at 0.062MPa. Any smooth fiber or line with a tensile strength of at least 25Nand with a diameter smaller than the perforation diameter could be used;smooth-surface fibers are necessary in order to break rather than cutthe web between the adjacent perforations. When looping the fiberthrough a single web of the device, care was taken to assure that onlythe web, and not any section the metal portion of the stent, was looped.The tensile tester was then run at a jaw separation rate of 100 mm/min(consistent with ISO 7198 requirements for suture retention strengthtest). Testing was performed at room temperature (24° C.). Web tensilestrength was determined from the peak force required to cause the singlesection of web to break.

Porosity index was also determined for stents provided with theperforated cover. This can be determined using the stent inspectionsystem as described above or by analyzing each component (stent andcover) separately. This is accomplished on a uniformly perforatedstructure by measuring the area of the nominal perforation andmultiplying by the total number of perforations. This is then divided bythe total area of the cover, subtracting this value from 100%, resultingin the cover area percentage. The stent area ratio can be determined byusing techniques commonly know in the industry (e.g., ASTM F2081).

Bend radius was determined using the method defined in ISO 7198 (1998,section 8.9).

All perforated cover-based device samples were built as described abovewith the following exceptions. For Samples A1 and A2, the perforatedcover was sized to fit on a 6 mm stent where perforated cover aperturesizes were approximately 0.40 to 0.45 mm with about 40 holes percircumference. For Samples B1 and B3, the perforated cover was sized tofit on a 6 mm stent where perforated cover aperture sizes wereapproximately 4.0-4.2 mm with 5 holes per circumference. For Samples C1and C3, the perforated cover was sized to fit on a 37 mm stent whereperforated cover aperture sizes were approximately 0.40 to 0.45 mm with220 holes per circumference. For Samples D1 and D3, the perforated coverwas sized to fit on a 37 mm stent where perforated cover aperture sizeswere 5.5 to 5.6 mm with 20 holes per circumference. For Sample E, theperforated cover was constructed using the same method as for Samples D1and D2, but the perforated cover was not attached to a stent. Allsamples had an approximate thickness of 40 microns, as determined by acalibrated snap gauge (Mitutoyo Model ID-C112EB on base Mitutoyo Code7004).

Samples were tested as described above. Sample A had porosity index of52% and a web tensile strength of 2.8N. The Sample A2 was bent around acalibrated pin with a diameter of 5.5 mm without kinking. Samples A1 andA2 had a web width of about 60 to 100 microns. Sample B1 had a devicetensile strength of 7.0N while Sample B2 had a web tensile strength of4.4N. Samples B1 and B2 had a porosity index of 78%. The Sample B3 wasbent around a calibrated pin with a diameter of 1.5 mm without kinking.Samples B1-B3 had a web width of about 100 to 120 microns. Sample C1 hada device tensile strength of 95.8N while Sample C2 had a web tensilestrength of 2.0N. Samples C1 and C2 had a porosity index of 53%. TheSample C3 was bent around a calibrated pin with a diameter of 12.8 mmwithout kinking. Samples C1-C3 had a web width of about 80 to 90microns. Sample D1 had a device tensile strength of 20.9N while SampleD2 had a web tensile strength of 4.7N. The Sample D3 was bent around acalibrated pin with a diameter of 6.4 mm without kinking. Samples D1-D3had a web width of about 120 to 200 microns. Sample E (perforated coveronly) had a device tensile strength of 21.0N. Sample D had a porosityindex of 82%.

Note that when testing the perforated cover alone (Sample E), thecover's tensile strength for this construct of 20 holes/circumference,corresponding to 20 webs/circumference, was 21N. Other constructs with10, 5, or 2 holes may be desirable (corresponding to 10, 5, and 2webs/circumference respectively), with estimated corresponding tensilestrengths of 10N, 5N, and 2N respectively.

While particular embodiments of the present invention have beenillustrated and described herein, the present invention should not belimited to such illustrations and descriptions. It should be apparentthat changes and modifications may be incorporated and embodied as partof the present invention within the scope of the following claims

1. An implantable stent comprising adjacent stent elements spaced apartlongitudinally wherein said stent elements are connected by a perforatedcover having a multiplicity of hexagonal apertures.
 2. An implantablestent comprising adjacent stent elements spaced apart longitudinallywherein said stent elements are connected by a perforated cover, saidperforated cover having a multiplicity of substantially hexagonalapertures when said stent is at a deployed diameter suitable forimplantation, said deployed diameter being larger than a compacteddiameter suitable for passage of the stent through a body conduit.
 3. Animplantable stent according to claim 2 wherein said aperture issubstantially rectangular when said stent has the compacted diameter. 4.An implantable stent comprising adjacent stent elements spaced apartlongitudinally wherein said stent elements are connected by a perforatedcover, said perforated cover having a multiplicity of apertures throughthe cover wherein the minimum aperture size is about 0.10 mm, whereinsaid stent including said perforated cover has a porosity index of atleast about 50%, and wherein said cover is a substantially integralmaterial.
 5. A cover for use with an implantable stent, said covercomprising an integral, non-woven sheet having a length and width and anarea defined as a the length multiplied by the width, said cover havinga multiplicity of perforations therethrough, wherein said perforationscollectively cover an area that is at least 50% of the sheet area,wherein said perforations have a minimum aperture size of about 0.10 mm.6. A cover according to claim 5 wherein said perforations have agenerally hexagonal shape.
 7. An diametrically expandable endoprosthesiscomprising a substantially open tubular stent and at least one flexiblenon-metallic strip, said strip oriented substantially longitudinally andconnected to the stent to limit foreshortening and elongation of thestent.
 8. A diametrically expandable stent comprising at least twocircumferentially oriented spaced-apart stent elements and at least onesubstantially longitudinally oriented flexible connecting stripconnecting adjacent spaced apart stent elements, said stent having alength between ends of the stent and a deployed circumference, whereinsaid stent has a tubular area comprising the stent length multiplied bythe deployed circumference and wherein at least 50% of the tubular areais open.
 9. A diametrically expandable stent according to claim 8wherein at least 60% of the tubular area is open.
 10. A diametricallyexpandable stent according to claim 8 wherein at least 70% of thetubular area is open.
 11. A diametrically expandable stent according toclaim 8 wherein at least 80% of the tubular area is open.
 12. Adiametrically expandable stent according to claim 8 wherein at least 90%of the tubular area is open.
 13. A diametrically expandable stentaccording to claim 8 wherein at least 93% of the tubular area is open.14. A diametrically expandable stent according to claim 8 wherein atleast 80% of the tubular area is open and three connecting strips areused.
 15. An expandable prosthesis comprising at least twolongitudinally spaced-apart stent elements, said stent elementscomprising a serpentine form having apices separated by relativelystraight segments, wherein alternating apices point in opposingdirections, and wherein the at least two longitudinally spaced-apartapices are connected by at least one substantially longitudinallyoriented flexible filament.
 16. An expandable prosthesis according toclaim 15 wherein said filament includes at least one transverselyoriented loop around at least one apex.